Yes, you are right. It does not really matter Exception 2 when it is fully taxable. Everything goes to 4b and nothing in 4a. So, there is no contradiction between pub 590-B and 1040 Exception 2. I wish they have just added that simple statement that if conversion is fully taxable ignore 4a, just go for 4b. I guess it is written for the purpose that you might have non-taxable portion and then total goes in 4a and only taxable portion in 4b. Now it all makes sense.
